A conventional so-called static particle size distribution measuring device calculates, as shown in the patent document 1, a particle size distribution based on the following expression (Expression 1).s=Kq  (Expression 1)
where “s” is a vector that expresses a space intensity distribution of a secondary light obtained from output signals of multiple light sensors arranged dispersedly around a particle group, “q” is a vector that expresses a particle size distribution, and “K” is a coefficient matrix that is uniquely determined by a physical property concerning a refractive index of the particle group and an arranged position of the light sensor.
Here, since what to be obtained is the vector (q) that expresses the particle size distribution and the vector (q) is in the right side of the expression, an inverse problem is to be solved.
Then, conventionally, the vector (q) (the particle size distribution) is calculated by the use of a previously determined single iterative solver such as, for example, the Twomey iterative method. The iterative solver is to first give an imaginary solution of the particle size distribution, to calculate an imaginary light intensity distribution by substituting the imaginary solution for the above-mentioned expression 1, to update the imaginary solution one by one so as to make the imaginary light intensity distribution approach the actually measured space intensity distribution (hereinafter also called as a real space intensity distribution) and to calculate the resultantly obtained imaginary solution as the particle size distribution.